Battery Life Calculator

Estimate how long a battery lasts from its capacity (mAh) and the device load current (mA), with an adjustable efficiency factor.

Inputs

Rated charge capacity printed on the cell or battery (milliamp-hours).

Average current the device draws (milliamps). Use the typical, not peak, draw.

Usable fraction of rated capacity after Peukert losses, voltage cut-off, and self-discharge. 100% = ideal; 70–90% is typical in practice.

Result

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How to use this calculator

  • Enter the battery capacity in mAh (printed on the cell, e.g. 3000 for a typical 18650).
  • Enter the average current your device draws in mA — use the typical running draw, not the peak.
  • Set the efficiency factor: 100% for a theoretical maximum, 80–90% for a healthy battery, 70% or lower for old/cold/high-drain conditions.
  • Read the estimated runtime in hours and the human-readable days/hours/minutes breakdown.

About this calculator

This calculator estimates how long a battery will power a device from two numbers you can read off the spec sheet: the battery's rated capacity in milliamp-hours (mAh) and the average current the device draws in milliamps (mA). Because a milliamp-hour is defined as the charge delivered by one milliamp flowing for one hour, dividing capacity by load gives the runtime in hours directly. Real batteries never deliver 100% of their rated charge to the load, so an efficiency factor (typically 70–90%) accounts for voltage regulator losses, the Peukert effect at higher discharge rates, self-discharge, and the fact that devices stop working before the cell is fully empty. Lower the factor for old cells, cold temperatures, or high drain.

How it works — the formula

t (hours) = Capacity (mAh) × η ÷ Load (mA) where η = efficiency factor (0–1) C-rate = Load (mA) ÷ Capacity (mAh)

A milliamp-hour is a unit of electric charge equal to one milliamp sustained for one hour (Q = I·t). Runtime is therefore charge divided by current. The efficiency factor η scales the rated capacity down to the charge actually delivered to the load under real conditions.

Sources: NIST — The ampere and the ampere-hour (SI unit of charge) · DigiKey — Battery Life Calculator (formula reference)

Worked examples

Example 1
3000 mAh cell, 150 mA load, 85% efficiency
Inputs:
capacity=3000, load=150, efficiency=85
Output:
3000 × 0.85 ÷ 150 = 17.00 hours
Example 2
2000 mAh cell, 100 mA load, ideal
Inputs:
capacity=2000, load=100, efficiency=100
Output:
2000 ÷ 100 = 20.00 hours
Example 3
5000 mAh power bank, 500 mA load, 80% efficiency
Inputs:
capacity=5000, load=500, efficiency=80
Output:
5000 × 0.80 ÷ 500 = 8.00 hours

Limitations

  • Assumes a constant average load; bursty or duty-cycled loads need an averaged current.
  • Does not model the discharge voltage curve — runtime to a device's specific cut-off voltage may differ.
  • The Peukert effect is approximated by the single efficiency factor, not modelled per-rate.

A planning estimate, not a guarantee. Verify critical applications (medical, safety) with a measured discharge test.

Frequently asked

What is the basic battery life formula?+
Runtime (hours) = battery capacity (mAh) ÷ load current (mA). This works because a milliamp-hour is the charge delivered by 1 mA over 1 hour, so the units cancel to give hours. Multiply by an efficiency factor to account for real-world losses.
Why multiply by an efficiency factor?+
A battery rarely delivers its full rated capacity to your load. Voltage-regulator losses, self-discharge, the Peukert effect (capacity drops as discharge current rises), and the device's low-voltage cut-off all reduce usable charge. An 80–90% factor is realistic for a healthy battery; use 70% or less for old, cold, or heavily loaded cells.
Does mAh capacity depend on voltage?+
mAh measures charge, not energy. Two batteries with the same mAh but different voltages store different amounts of energy (Wh = V × Ah). If your device's current draw is specified at its operating voltage, the mAh ÷ mA formula is valid. To compare energy, convert to watt-hours.
What is the C-rate shown in the results?+
The C-rate is the discharge current relative to capacity: load ÷ capacity. A 1C rate drains the battery in roughly one hour; 0.5C in two hours. High C-rates (above ~1C) increase Peukert losses, so you should lower the efficiency factor accordingly.
Can I use this for a phone or laptop battery?+
Yes for a rough estimate, but device power management makes the draw highly variable. A phone idling draws far less than during video or GPS use. Estimate an average mA from the battery's Wh rating divided by voltage, or measure with a power meter, for better accuracy.
Why is my real runtime shorter than the calculator says?+
Common causes: the device draws more than your assumed average, the battery is aged and holds less than its rated capacity, the temperature is low (cold reduces usable capacity), or the discharge rate is high enough that the Peukert effect matters. Reduce the efficiency factor to model these.

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